This invention relates in general to a stent and more particularly to a stent that is designed to be particularly flexible so as to navigate tortuous zones of the vascular system while retaining sufficient structural integrity to perform an effective stent function.
A wide variety of stents have been proposed and a number are in use. A major function of stents is as vascular stents which are implanted in the vascular system to hold back obstructions (such as plaque and thrombus) so as to provide ready passage of blood through the lumen. These are employed in percutaneous transluminal vascular angioplasty.
Many stents lack sufficient flexibility to navigate tortuous lumens or to be adequately positioned within a tortuous lumen. This is one of the major limitations on the stent shown in the Palmaz U.S. Pat. No. Re 4,739,762.
There are other stents which have substantial flexibility that permit navigating through tortuous lumens such as the stent described in U.S. Pat. No. 5,217,483. issued to Allen J. Tower. The Tower stent uses a sinusoid shaped wire to form the stent. This geometry is naturally more flexible than articulated or expanding segment stents. Such a similar construction is also described in U.S. Pat. No. 5,019,090 by Pinchuk, and in U.S. Pat. No. 4,886,062 by Wiktor. However, such stents have a tendency to expand in a longitudinal direction, particularly when placed in a tortuous lumen. The result is to lose coverage and thus fail to prevent restenosis in the uncovered areas.
When certain stents are placed within a curve and expanded, gaps appear in the stent structure on the outside of the curve. If the stent is expanded in an area with calcified plaque, the plaque can separate the stent segments allowing the plaque to protrude into the lumen. This is called plaque prolapse.
Another problem with some stent designs is that they are weak and can be easily disrupted by guide wire and catheter manipulation. Stents that can negotiate tortuous curves are not good for supporting lesions in these curves because of the weakness which allows the flexibility.
The need to obtain a stent which is highly flexible but which does not expand or contract in a longitudinal direction when placed in position is recognized and has been addressed for example in U.S. Pat. No. 5,843,168 issued to Kenny L. Dang.
However, experience shows there is difficulty in obtaining the appropriate lumen sidewall coverage in a highly flexible stent that maintains its longitudinal structural integrity.
The trade-off between flexibility and structural integrity or robustness is unsatisfactory in many applications. What is required is a structure or geometry for the stent which preserves a high degree of flexibility with a high degree of longitudinal structural integrity. In spite of many proposals and approaches, there is a present need for a stent that achieves a better combination of these two operational requirements.
Accordingly, it is a primary purpose of this invention to provide a stent that has an enhanced combination of flexibility and longitudinal structural integrity.
It is a related purpose of this invention to provide a device which achieves the above result in a fashion that avoids complex insertion and placement procedures.
It is a further object of this invention to provide a stent achieving the above objectives which provides adequate interior lumen area coverage when placed in position.
Another object is to provide a flexible stent which will have sufficient radial stiffness (hoop strength) to maintain its expanded state yet avoid longitudinal collapse when the placement balloon is withdrawn.
A further object is to provide a stent with a minimal profile so that small diameter body lumens can be traversed and accessed.